Mexico City residents packed buses and subway cars and many walked or biked to work on Wednesday, as authorities barred millions of vehicles from the streets due to a pollution alert.

Under new regulations imposed after the capital recently experienced its worst air quality crisis in over a decade, the Phase 1 alert was declared when smog levels hit one-and-a-half times acceptable limits on Tuesday.

Pollution levels were down somewhat on Wednesday, but Martin Gutierrez, executive coordinator for the Environmental Commission of the Megalopolis, told broadcaster Milenio TV that the alert would remain in place until at least 5pm. A dense haze obscured the volcanic mountains that ring the city.

On any given day a fifth of all cars are supposed to stay off the streets, but that doubles during a Phase 1 alert. Until recently, newer and cleaner cars were exempt from the ban.

With two million of the capital's cars ordered out of circulation, or 40%, fares were temporarily waved for public transportation. Buses crawling along the central Reforma boulevard were crowded during the morning rush hour.

"Public transport is not sufficient," said Martin Colin, a 53-year-old taxi driver, pointing to a bus jammed with standing commuters. "They look like little sardines. The buses are full, full."

Traffic was noticeably lighter in some parts of the sprawling capital. But Mr Colin, whose morning fares had already taken him to diverse parts of the city, said many of the usual bottlenecks were as bad as ever.

"It's the same," he said.

The vehicle ban does not apply to public or private buses - something that has drawn criticism since many privately operated minibuses are notorious for belching thick black smoke.

Taxis prowled the streets in visibly greater numbers, and finding an unoccupied cab to hail was more difficult than usual. Dispatchers reported increased demand, and Uber users complained of surge pricing several times higher than normal.

Many residents turned to Mexico City's expanding bicycle-share programme for the commute.

Factories across greater Mexico City were ordered to cut emissions by 30 to 40%, and gas stations without vapour recovery systems were told to close.

The anti-pollution measures announced last month last until the end of June, when the rainy season begins. Authorities say they are working on longer-term policies to be announced in the future.